Apple snails belong to the phylum Mollusca, class Gastropoda (snails), subclass
Prosobranchia, order Caenogastropoda (Mesogastropoda in older literature), superfamily
Ampullarioidae, family Ampullariidae (Apple snails). The whole apple snail family
(Ampullariidae) is divided in 7 (or 10, depending of the authority followed)
genera [click here to see their characteristics]
and contains about 120 different species*. However, much work has to be done
to identify all species, to rule out the misidentified species and to remove
the synonyms. A list with reported geographical locations of several species
is available: [Reported geographical locations
160K].

* New insights in the phylogenetic relationships reduce the
actual number of species drastically. See the Pomacea
page.

For those who want to identify the apple snails derived
from the aquarium trade:
First of all it's important to make sure that you are dealing with an apple
snail (obvious). If you are not sure about this, you can check
it out here.
In general, it's quite simple: if you bought your animal from a pet shop, you
probably have a Pomacea diffusa
apple snail or a snail from the Pomacea canaliculata - insularum group.
The latter is a group of very similar and closely related species. In most cases,
the offered snail is a Pomacea insularum
or a Pomacea canaliculata
snail, but occasionally Pomacea haustrum
and possibly other species from the canaliculata group, like Pomacea
maculata might be available
as well.
For the snails of the Pomacea canaliculata - insularum group, it's quite
hard to distinguish the species as the external differences are subtle and the
variation within species is high. A possible way do distinguish these snails
is the colour of the eggs*. one should also keep in mind that hybrids between
several closely related species might occur. This cannot be ruled out in invasive
species and those available in the aquarium trade.

* This is based on reported egg colours, and it could well
be that even the egg colour varies withing one species. Also the food composition,
the development stage of the eggs and the humidity of the air can affect the
egg colour.

When the snail has a flat shell and has a size over 2.5 cm (1 inch) with or
without dark stripes, you are the owner of a Marisa cornuarietis
apple snail (Giant ramshorn snail). This snail is not always recognized as an
apple snail due to its different appearance.
Less common, but also occasionally available in the aquarium trade (mainly in
the US) is Pomacea paludosa (Florida apple snail).
Below is a quick and dirty guide to identify them based on the shape of the
shell. Once more: Ignore the colour of your snails when you try to determine
the species by comparing them with pictures. Many colour variations (shell and
body) exist within a single species (see photograph on the right) and in the
'Colour genes' section.

Pomacea diffusa):
flat shoulders and 90° sutures (scalariform).
The flat shoulder, however, becomes less apparent in the last whorls of
the shell. Pomacea bridgesii
is not known to be available in the aquarium trade.

Pomacea canaliculata
and Pomacea insularum:
indented sutures, less than 90°angle.
This shell is more globose (rounder) than the Pomacea diffusa shell.

Pomacea paludosa:
Almost flat sutures with an over 90° angle, which gives the snail a cone
shaped shell top (spine).

Sometimes other apple snail species pop-up in the trade, mostly imported
species. In these cases, it can be quite difficult to identify them. Try to
find out from which region they come as this makes identification much easier.
If you need help to identify an apple snail: feel free to
and take some pictures of the snail in such way that the following thing are
visible: shell opening (aperture), whole shell
shape (including the top), the umbilicus (or
measure the size), the sutures of the shell and if available the shell door
(operculum). Also measure the size of the shell and if available describe
if the operculum is horny (flexible) or calcified.
Have a look at the Pomacea flagellata
section to see how a useful picture of the shell looks like.

Notes on the family name
(Ampullariidae):
There is a lot of confusion in the nomenclatura of apple snails: the family name is often
referred to as Ampullariidae, sometimes as Pilidae (invalid).
In 1758 Linnaeus, who regarded the Ampullariidae as terrestrial, referred Pila
ampullacea to the genus Helix.
His assumption was based on the fact that he never saw a living example, he
only got a shell and thought it to be a terrestial snail.
In 1797 Humphrey attempted to erect the generic name Pomus and described 5
species in this genus. But according to Dall (1904) his work was published anonymous, was
not for sale, didn't bear the name of the author or the name of the publisher and is
invalid under the rules of nomenclature. This all makes the generic name Pomus
invalid.
In 1798 Röding proposed the generic name Pila (with Helix ampullacea as
the type species designated by Dall (1904)).
This genus description did fulfil the requirements of the nomenclature rules, although it
was later restricted to the Old World Ampullariidae, with the exception of the West
African genera Saulea (Gray, 1867) and Afropomus (Pilsbry and Bequaert,
1927).
In 1799 Lamarck used the name Ampullaria for a single species (Helix
ampullacea), making it the type species for Ampullaria by monotypy. However,
Dall (1904) designated Helix ampullacea as the type species of Pila
rendering Ampullaria Lamarck, 1799 a junior objective synonym of Pila
Röding, 1798, despite his designation of Nerita urceus Müller, 1774 as the type
of Ampullaria.
In March 1810 Perry created the genus Pomacea (type species Pomacea
maculata) in which the American
species were placed.
In the same year (1810), a few months after Perry's publication, Montfort introduced the
generic name Ampullarius as an unjustified emendation of Ampullaria.
The generic name Ampullarius is a junior subjective synonym for Pomacea
Perry, 1810 (accepted by the ICZN, OPINION 13). Unfortunately Ampullarius is
still incorrectly used in the aquarium trade.
In 1824 Gray introduced the family name Ampullariidae (type genus Ampullaria
Lamarck, 1799), including both the Old and New World taxa.
Preston (1915) introduced the family name Pilidae (type genus Pila Röding, 1798)
to replace the family name Ampullariidae, acknowledging the generic name Ampullaria
Lamarck, 1799 a junior synonym of Pila Röding, 1798.
However to correctly (under the rules of the nomenclatura) replace the family name
Ampullariidae with Pilidae a general acceptance is needed (ICZN Code article 40(b)), but
as most authors preferred Ampullariidae above Pilidae this requirement hasn't been
fulfilled. The family name Ampullariidae Gray, 1824 should therefore be considered as the
valid name and this has been accepted by the International Commission on Zoological
Nomenclature OPINION 1913, placing Pilidae Preston, 1915 on the Official Index of Rejected
and Invalid Family-Group Names in Zoology.